The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Kids spend too long watching screens

Children are spending more than half their spare time in front of a screen after embracing smart device technology, a study has found.

Tablets and smartphones have joined TV as the major entertainers of children, the New Generations study found.

Kids spend more than half their spare time in front of devices, sparking concerns children are not spending enough time interacting with real people and getting exercise.

More than half of the children surveyed had a smartphone and two-thirds of children have used an app device, the study found.

On average children are accessing 7.1 apps a month.

Children are abandoning gaming consoles - their use has dropped 32 per cent since the last survey in 2011.

Psychologist Professor Matthew Sanders said it was "extremely concerning" that parents are allowing their kids to spend so much time in front of a screen.

"Kids need to be able to amuse themselves in a whole variety of ways," he told AAP.

"I just reject he idea that because lots of kids are doing it, it is necessarily healthy or desirable.

"What is a much more preferable alternative is that children have closely monitored screen time and participate in activities that are age appropriate."

Prof Sanders said children should not spend more than two hours a day in front of a screen and should spend time doing things such as physical activity, talking to friends in person and reading.

But he said there is no doubt there are highly engaging applications on smart technology devices that have a very positive purpose.

Peter Hammer, a research manager for Turner Broadcasting System Asia Pacific, who conducted the study, said the combination of internet enabled devices was creating powerful ways to connect with children.

"Apps have redefined the way that parents are entertaining kids."

He said the rapid explosion of smart devices has changed the way children are accessing the internet and playing games.